Chain structure, linear viscoelasticity and extensional rheology of poly(4-vinyl biphenyl-stat-styrene) statistical copolymers

A series of statistical copolymers of 4-vinylbiphenyl and styrene were prepared via free radical polymerization to investigate the effect of the comonomer ratio on their microstructure and rheological properties. The glass transition temperature of the copolymers follows a nonlinear monotonic decreasing dependence with increasing styrene content which is well described by the Gordon-Taylor equation. WAXS measurements reveal that both the backbone to backbone and the inter-phenyl distances (d(1) and d(2), respectively) are decreasing functions of the styrene content in the copolymers. The apparent monomeric friction coefficient, calculated from dynamic frequency sweeps in the linear viscoelastic regime, decreases exponentially with the styrene content in the copolymer. We postulate that the strong effect of the copolymer composition on the friction coefficient is due to the larger overlapping distance between phenyl rings of adjacent polymer chains (d(overlap)), giving rise to stronger aromatic pi-pi interactions, which in turn results in stronger friction between adjacent chains. The increase in d(overlap) and monomeric friction coefficient also leads to a significant increase in the extensional strain hardening measured during extensional startup flow measurements.